Despite intensive effort to discover drugs that may be of value in the systemic treatment of human immunodeficiency virus (HIV) infections, such infections have been singularly resistant to chemotherapy. The intracellular and intimate relation to nuclear metabolism of virus reproduction makes it difficult to destroy a virus without irreparable damage to the host cell.
The discovery of the antiviral activity of vidarabine (9-.beta.-D-arabinoturanosyladenine monohydrate) has led to the preparation of a large number of synthetic nucleosides. To date, only one synthetic nucleoside, 3'-azido-3'-deoxythymidine has been approved for treating certain AIDS patients, but it is a palliative, not a cure. ##STR2##
Although AZT is specifically active against retroviruses, its use has led to side effects, including anemia, headache, confusion, anxiety, nausea and insomnia. The nucleoside analog, 2',3'-dideoxycytidine (DDC), exhibits an in vitro TI.sub.50 of ca. 300 against HIV and may exhibit fewer side effects than AZT, but may also be eliminated more rapidly from the body. ##STR3##
The synthesis of adenine ("6-amino-purine") nucleoside analogs in which the pentose sugar has been replaced with tris(hydroxy)-substituted cyclopentyl residues has yielded compounds with substantial cytotoxic and antiviral activity. For example, the carbocyclic analog of vidarabine, cyclaridine, is highly active against HSV-2, but exhibits a low therapeutic index (TI.sub.50 =10) against HIV in vitro. Likewise, the carbocyclic analog of AZT is inactive against HIV. Therefore, it is clear that the structure-activity relationships between the variously substituted carbocyclic nucleosides which have been prepared and tested remain ill-defined.
Thus, a substantial need exists for chemotherapeutic agents effective to protect mammalian cells against infection by viruses such as HSV-2, HIV, varicella-zoster, vaccinia, human cytomegalovirus (HCMV) and the like.